1. Field of the Invention
The invention relates to a vehicle lifting device comprising a number of columns along which carriages are movable by means of guide means. These carriages bear vehicle supporting means which can engage beneath a vehicle, such as for instance a car.
The carriage is connected to lifting means which further engage on the column, whereby the carriage can be moved upward relative to the column in order to move upward the vehicle supported by the vehicle supporting means. The lifting device further comprises synchronization means which ensure that all carriages of all columns run synchronously, whereby the supported vehicle can be moved up and downward in the same relative position.
2. Description of the Prior Art
Such vehicle lifts described above are generally known and occur in for instance two-column and four-column embodiments.
In a known four-column lift with two wheel tracks which are each supported by two columns, it is known to effect the synchronization of the two lifting means engaging on the same wheel track by making use of hydraulic cylinders connected in series. There is one drive motor therein which supplies the hydraulic power for the lifting means of the two columns. Hydraulic lines are therefore arranged along the wheel track which mutually connect the hydraulic cylinders of the two columns in the correct manner.
The maximum allowable load bearing capacity of such a lift is determined by the heaviest vehicle with a short wheel base relative to the wheel track length which can still be safely lifted when it is arranged in a most unfavorable position, i.e. with the most heavily loaded vehicle axle as far as possible to the end of a wheel track. Due to the series connection of the hydraulic cylinders, the columns furthest removed from the vehicle will have to supply only a small lifting power in such a most unfavorable loading situation. The lifting means of the most heavily loaded columns supply the greatest part of the lifting power.
In the lifting device according to the invention the lifting means of each column are advantageously provided with their own motor. The connection present in the described prior art between the two co-acting columns can hereby be omitted, so that a simpler construction is achieved. A significant additional advantage is moreover achieved, i.e. that with the same dimensioning as in the prior art the maximum lifting capacity of the bridge is higher. The total lifting capacity of the bridge is equal to the sum of the maximum lifting capacity of all columns together. In the described prior art the maximum capacity of the bridge equals the sum of the load of the heavily loaded and lightly loaded columns in the described most unfavorable loading situation.
Nor in the prior art lift can a heavier vehicle be lifted when the center of gravity of the vehicle is properly positioned in the middle of the column, because no provisions are made to make the lifting capacity dependent on the load on the columns.
A very favorable further development is characterized by the columns having the same form. This enables rationalization of production. The cost price of the lift according to the invention can hereby be reduced in advantageous manner.
An electric motor coupled to a hydraulic pump supply the xe2x80x9cliftxe2x80x9d to the device. Because use is made of an electric motor as power source, control of the different columns can take place in suitable manner by electrical or electronic means.
For the lowering of a vehicle a pressure-compensated descent volume flow control valve is preferably connected in parallel to the hydraulic pump. The volume flow is herein substantially adjusted so as to be equal to that of the pump, so that ascent and descent of the vehicle lifting device takes place at roughly the same speed. Through the use of a pressure-compensated volume flow control valve the descending speed of the carriages of each of the columns will be practically the same irrespective of the load, so that the synchronization means have to provide little or no compensation.
The invention is applied in suitable manner in a vehicle lifting device having columns which in pairs bear two wheel tracks connected to the carriages. The advantage of increasing the maximum lifting capacity is particularly manifest in such a lift because the consequences of the difference in the load on the columns co-acting with one wheel track can be relatively serious.
The invention also has a support resting on the floor surface in a low position of the carriage. When a heavier vehicle corresponding to the higher lifting capacity is driven onto the lift, the higher load on the carriages and the columns on the drive-on side of the lift is absorbed by the support. Once the heavier vehicle has been positioned on the bridge, the load on the relevant carriages and columns is then reduced to the usual level. In this way it is thus possible to suffice with relatively lightly dimensioned columns.
Preferably the support described above is situated under the wheel track. Practically the whole of the load occurring during driving-on is transmitted directly to the floor surface, so that in this situation the carriages and columns remain practically unloaded. When the wheel tracks are adjusted to adapt to the tread width of the vehicle for lifting, the support moreover remains in each case precisely at the correct position for good transmission of the load.
In a preferred embodiment, flow control valves react to position sensors on the carriage resulting in a switching on of the volume control valve in the column having the highest position. By switching on the volume flow control valve of each of the columns wherein the carriage has the highest position, the ascending speed of the carriage in question is decreased or the descending speed increased, whereby the difference in measured heights is reduced.
In further preference, the position sensors in the carriage can deactivate the motors if a given difference in an individual carriage position occurs. This safety provision comes into operation as soon as a carriage of one of the columns rises no further, for instance due to overloading.
Also the descent volume control valves can be deactivated when a maximum allowable difference in the individual position sensors is reached. This ensures that no hazardous situation can occur, for instance when during descent of the bridge the wheel tracks or the vehicle encounter an obstacle, for instance a forgotten support.
In this preferred embodiment of the protection means, in a situation where the descent volume flow control valves are deactivated, the motors will also have already been deactivated. To nevertheless enable removal of the cause of the activation of the descent protection means, for instance the above mentioned forgotten support, an operating member is provided. The motors can be set into operation again by actuating this operating member, whereby the carriages move upward and the obstacle can be removed.
If an even greater difference occurs in the position determined by the position sensors, the control means will preferably fully disable the device.
The invention will be further elucidated in the following description of an embodiment with reference to the annexed FIGURE.